Make Try Dart tests run on Firefox.

dart/site/try/src/interaction_manager.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library trydart.interaction_manager;
 
 import 'dart:html';
 
 import 'dart:convert' show
     JSON;
@@ skipping 37 matching lines @@
     info,
     warning;
 
 import 'html_to_text.dart' show
     htmlToText;
 
 import 'compilation_unit.dart' show
     CompilationUnit;
 
 import 'selection.dart' show
-    TrySelection;
+    TrySelection,
+    isCollapsed;
 
 import 'editor.dart' as editor;
 
 import 'mock.dart' as mock;
 
 import 'settings.dart' as settings;
 
 const String TRY_DART_NEW_DEFECT =
     'https://code.google.com/p/dart/issues/entry'
     '?template=Try+Dart+Internal+Error';
@@ skipping 153 matching lines @@
   }
 
   void onModifiedKeyUp(KeyboardEvent event) {
   }
 
   void onUnmodifiedKeyUp(KeyboardEvent event) {
     switch (event.keyCode) {
       case KeyCode.ENTER: {
         event.preventDefault();
         Selection selection = window.getSelection();
-        if (selection.isCollapsed && selection.anchorNode is Text) {
+        if (isCollapsed(selection) && selection.anchorNode is Text) {
           Text text = selection.anchorNode;
           int offset = selection.anchorOffset;
           text.insertData(offset, '\n');
           selection.collapse(text, offset + 1);
         }
         break;
       }
     }
 
     // editor.scheduleRemoveCodeCompletion();
@@ skipping 548 matching lines @@
     StringBuffer buffer = new StringBuffer();
     int selectionOffset = htmlToText(node, buffer, selection);
     Text newNode = new Text('$buffer');
     node.replaceWith(newNode);
     if (selectionOffset != -1) {
       selection.anchorNode = newNode;
       selection.anchorOffset = selectionOffset;
     }
   }
 }